Myanmar's civilian President, Htin Kyaw, has resigned with immediate effect, "in order to take rest from the current duties and responsibilities", his office has announced in a statement posted on Facebook.
According to the country's constitution, the more senior of two vice-presidents will stand in as president until a new leader is elected by Parliament within seven working days.
This means Myint Swe, who was the military's appointment for Vice-President, will become acting president.
The president is the head of state and government in Myanmar, and under the constitution has far-reaching powers.
However, Htin Kyaw's role was more ceremonial because Aung San Suu Kyi has been Myanmar's de facto leader since April 2016.
A constitution drafted by the former junta bars Nobel Peace Prize laureate Ms Suu Kyi from the top office and so she hand-picked Htin Kyaw, a close ally of hers, to become president.
Aung Shin, a spokesman for Ms Suu Kyi's National League for Democracy (NLD) party, said Htin Kyaw, 71, had stepped down due to ill-health.
The President's office did not give a reason for Htin Kyaw's resignation.
"The next president will be an NLD member or one who suits with NLD policy," Mr Aung Shin said.
"The current vice-president cannot be the next president, according to the constitution."